Various systems are known for optically sensing the underbody of a vehicle. These systems are marketed in the field of safety engineering. See, for example, the webpages at the .com websites available at the www address of “gatekeepersecurity” and the http address of “advanced-detection-technology.com.” In addition to an optical sensing unit, these systems as a rule have further diagnostic units, e.g., for vehicle identification or for detecting hazardous materials, persons, etc.
U.S. Pat. App. Pub. No. 2006/0170768 describes a mobile system for inspecting the underbody of the vehicle, having a platform on which a plurality of sensors are mounted, and a data-analysis device to evaluate the data supplied by the sensors. The sensors scan the underbody of the vehicle by moving relative to the platform.
U.S. Pat. App. Pub. No. 2007/0040911 describes a system for observing the underbody of a vehicle using a single camera which is set up to record an image that encompasses the entire width of the underbody of the vehicle. Along its optical axis, the camera has a viewing distance from the underbody of the vehicle which is greater than the Euclidean distance between the camera and the point at which the optical axis of the camera meets the underbody of the vehicle.
Generally, the sensor units or scanning units of known systems for sensing the underbody include a camera whose visual beam is deflected via a mirror onto the underbody of the vehicle.
Depending on the alignment of the mirror, the representation of the vehicle underbody may be distorted. If the visual beam strikes the underbody of the vehicle orthogonally, the underbody is able to be captured directly and without distortion. Otherwise, the perspectively distorted images of the underbody must be rectified by calculation in a further processing step.
Therefore, the following demands are to be placed on the sensing device of an optical scanning unit:                complete coverage of the width of the underbody with homogeneous spatial resolution;        distortion-free imaging of the underbody; and        adequate photo frequency as a function of the overtravel speed        
In addition, in order to generate an overall image, an evaluation method is needed which joins the individually acquired images together without overlap.